[c-nsp] Load Balancing, Part II

[Oliver Boehmer (oboehmer)]

Roy <> wrote on Friday, December 30, 2005 5:42 PM:

> Oliver Boehmer (oboehmer) wrote:
> 
>>> Thanks to all who responded on my load balancing question on
>>> different protocols.  Now onto part II of the quiz :-)
>>> 
>>> I have two routers R1 and R2 attached to a single customer C.  The
>>> routers are connected via BGP with R1 and R2 as members of a
>>> confederation.  I would like to load balance between the path R1-C
>>> and R1-R2-C. 
>>> 
>>> BGP doesn't seem to want to install both routes.  The AS length is
>>> the same since the confederation AS (65002) gets removed.  How can
>>> one determine why it won't multipath?
>> 
>> I'm not entirely sure if we're supposed to do multi-path in this
>> situation as confed paths are treated as "internal" in the comparison
>> (see Step 7 in http://www.cisco.com/warp/public/459/25.shtml), and
>> eiBGP multipath only works for vpnv4 paths.
>> 
>> You could try to configure the hidden command "bgp bestpath
>> multipath-relax as-path" (if it is available in the release you are
>> running) to see if it makes a difference, but I don't think it will
>> help. 
> 
> I don't have the command.  I am on 12.2(15)T5.

ok.. I made an error, the command is called "bgp bestpath as-path
multipath-relax", it is available in 12.3(12) and later.. But I just 
> 
> I did look up
> 
>
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a008009
4431.shtml#bgpmpath
> 
> Which could seem to insinuate that confed-external paths are treated
> as external for multipath
> 
> "The path should be learned from an external or confederation-external
> neighbor (eBGP)."

correct, but before (same document) it says in step 7:

: Prefer eBGP over iBGP paths.
: 
: If bestpath is selected, go to Step 9 (multipath).
:
: Note: Paths that contain AS_CONFED_SEQUENCE and AS_CONFED_SET 
: are local to the confederation. Therefore, these paths are 
: treated as internal paths. There is no distinction between 
: Confederation External and Confederation Internal.

if you had multiple confed paths, we would consider them for multipath,
correct, but the confed path never makes it as a multipath candidate.

	oli